Method for the treatment of dusts and concentrates

ABSTRACT

A method for the treatment of a mixture of lead dusts and pyritic concentrates said method including the steps of: 
     a) feeding a mixture of lead dust and pyritic concentrate to a furnace containing molten slag; 
     b) smelting the mixture by injecting air or oxygen enriched air into the slag by means of a submerged lance to produce a lead bearing slag; and 
     c) recycling fume to the furnace. 
     A method for the treatment of lead dusts, without pyritic concentrate, said method including the steps of: 
     a) feeding lead dust to a furnace containing molten slag; 
     b) smelting the dust by injecting air or oxygen enriched air into the slag by means of a submerged lance to produce a lead bearing slag; and 
     c) recycling fume to the furnace.

This invention relates to a method for the treatment of dusts and concentrates particularly, the invention relates to a method for the treatment of lead dusts and low lead pyritic concentrates.

The invention has been developed primarily for use with lead dusts and low lead pyritic concentrates and will be described hereinafter with reference to such materials. However, it will be appreciated that the invention is not limited to this particular field of use.

Lead dusts are a by-product of conventional lead smelting and, in the past, these dusts had to be recycled many times to recover the lead. This increased the heat load on the smelter and reduced its capacity.

It is an object of the present invention to provide a method of smelting lead dusts and low lead pyritic concentrates which overcomes or substantially ameliorates the problems of the prior art.

In accordance with the present invention there is provided a method for the treatment of a mixture of lead dusts and pyritic concentrates said method including the steps of:

a) feeding a mixture of lead dust and pyritic concentrate to a furnace containing molten slag;

b) smelting the mixture by injecting air or oxygen enriched air into the slag by means of a submerged lance to produce a lead bearing slag; and

c) recycling fume to the furnace.

Also in accordance with the present invention there is provided a method for the treatment of lead dusts, without pyritic concentrate, said method including the steps of:

a) feeding lead dust to a furnace containing molten slag;

b) smelting the dust by injecting air or oxygen enriched air into the slag by means of a submerged lance to produce a lead bearing slag; and

c) recycling fume to the furnace.

Preferably, steps a), b) and c) of both of the above aspects of the invention are continuous.

In a preferred embodiment, coal, coke, oil or natural gas, or any combination thereof, may be used as a fuel.

In a further preferred embodiment, depending upon the gangue and ash content of the feed mixture, it may be necessary to add flux to produce a fluid slag.

The fume is preferably recycled to the furnace via a baghouse. Most preferably, the baghouse is a dedicated unit with an integral fume return system.

The preferred operating temperatures range from 1150° to 1250° C. with 1170° to 1190° C. as the most preferred range.

Preferably, the slag will contain less than 2% sulphur and the process offgas will have an SO₂ concentration of greater than 4% to aid SO₂ recovery at an acid plant.

Preferably the slag will have a composition in the range 20 to 50% Pb, 10 to 20% Fe, 5 to 15% SiO₂ and 2 to 5% CaO, with typical residual impurities of up to 10% Zn, 3% Al₂ O₃ and 3% MgO.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying FIGURE, in which:

FIG. 1 is a process flowsheet in accordance with the method of the invention.

The ISASMELT process is based on submerged gas injection into melts via a top entry submerged SIROSMELT lance to produce turbulent baths in which high intensity smelting or reduction reactions can occur.

Lead dust, low lead pyritic concentrate, coke breeze and flux are mixed in a pug mill with sufficient water to form a moist agglomerate feedstock. The feedstock is continuously fed to a furnace by conveyor and dropped directly into a molten slag bath where it is oxidized by air injected down a lance. The slag is tapped either continuously or batch-wise and the offgas cooled by a water spray directly above the furnace, as shown schematically in FIG. 1.

In the following examples two feed types are considered:

Mix A comprised 54.7% of lead sinter plant recycle dust, 12% of lead bearing copper converter dust and 33.3% of low lead pyritic concentrate. The low lead pyritic concentrate was included to increase the SO₂ content in the offgas.

Mix B comprised 82% of lead sinter plant recycle lead dust and 18% of lead bearing copper converter dust.

The material compositions are listed in Table 1.

                                      TABLE 1     __________________________________________________________________________     FEED MATERIALS USED DURING THE 250 Kg TESTWORK                                   NOMINAL CHARGE MIX     FEED MATERIALS (wt %)         Mix A         (wt %)                  Copper                        Low Lead   54.7% Recycle Dust                                                 Mix B     Sinter Plant Converter                        Pyritic                               Silica                                   12% Converter Dust                                                 82% Recycle Dust     Recycle Dust Dust  Concentrate                               Flux                                   33.3% Pyritic Concentrate                                                 18% Converter     __________________________________________________________________________                                                 Dust     Pb    32.20  30.10 13.50  --  25.72         31.82     Zn    5.10   7.60  7.60   --  6.23          5.55     Fe    8.49   1.50  24.40  1.0 12.95         7.23     Cu    1.90   2.50  2.00   --  2.01          2.01     S     12.40  13.10 33.00  --  19.34         13.53     S/SO.sub.4           4.91   12.30 1.80   --  4.76          6.24     SiO.sub.2           8.10   1.30  5.00   92.5                                   6.25          6.88     Al.sub.2 0.sub.3           1.70   0.20  0.60   2.5 1.15          1.43     CaO   5.10   3.20  1.10   0.5 3.54          4.76     MgO   0.50   0.20  0.20   0.3 0.36          0.45     As    1.20   4.10  2.00   --  1.81          1.72     Sb    0.80   0.80  0.70   --  0.77          0.80     Cd    0.76   1.10  0.02   --  0.55          0.82     Bi    0.11   1.48  0.05   --  0.25          0.36     Ag (ppm)           1520   270   4450   --  2340          1290     Au (ppm)           31.0   trace 7.2    --  19.0          26.0     Moisture           7.3%   14.4% 5.7%    4.0%     Proximate analysis Coke Breeze (5.5% moisture)                  86.80%                        fixed carbon                  4.20% volatile matter                  8.59% ash                  0.46% moisture     __________________________________________________________________________

EXAMPLE 1

Testwork on the lead dusts and low lead pyritic concentrates was carried out in a 250 kg ISASMELT test furnace. A total of 1780 kg of lead sinter plant recycle dust, 380 kg of lead bearing copper converter dust and 550 kg of low lead pyritic concentrate were smelted in 17 runs over 29 hours operating time.

The 250 kg ISASMELT test furnace has an internal diameter of 380 mm and is 1.5 m high and lined with chrome-magnesite refractories. Air is injected into the bath via a 40 mm nominal bore lance. Feed is added via a variable speed conveyor and drops directly into the molten slag bath. The offgas passes through an evaporative gas cooler and is cleaned in a reverse pulse baghouse.

The feed materials were mixed with coke breeze and pelletized prior to each run. Recycled fume was added during the run at a rate approximating the fume rate measured at the baghouse. Silica flux was also added to the Mix A material.

At the beginning of each series of runs, lead slag was fed into the furnace and melted to form a starting bath. The dust/concentrate mixture was then fed into the furnace at a controlled rate and smelted. Both normal air and air enriched to 35% O₂ were used. Oil was used as a supplementary fuel to the coke breeze.

During each run, slag samples were taken periodically from the furnace and the fume rates were measured at the baghouse. Operating temperatures ranged from 1150° to 1250° C., with 1170° to 1190° C. providing optimum conditions. At the conclusion of each run the slag was tapped into moulds and weighed.

The operating conditions and results from this testwork are tabulated in Table 2. Product assays from this testwork are given in Table 3.

    TABLE 2              AVERAGE  STARTING BATH NEW CHARGE ADDITIONS  TOTAL ACTUAL NEW      SMELTING FINAL  Initial  MATERIAL CHARGE Dry % of New Material NOMINAL      RUN SMELTING MATERIAL RATE BATH RUN Initial Depth (Dry %) Silica  Return M      OISTURE TIME TIME SMELTED (kg/hr) DEPTH NUMBER Charge (mm) D & L* EP**      Cons Flux Coke Fume CONTENT (min) (min) (kg, dry) (dry) (mm)        1 90 kg High 130 54.8% 11.2% 34.0% 4.3% 12.8% -- 13.0% 195 195 310 95      430 Mix A Lead Slag          ie 100% 27/2/89 2 Continuing 100 54.8%      11.2% 34.0% 4.3% 12.8% -- 13.0% 150 150 246 98 450 Mix A from Run 1          ie 100% 27/2/89 3 70 kg Run 2 Slag + 130 54.8% 11.2% 34.0% 4.3%      15.3% 12.2% 15.0% 205 50  64 77 220 Mix A 30 kg High 1/3/89 Lead Slag            ie 24%  4 100 kg High 140 54.8% 11.2% 34.0% 4.3% 15.3% 12.2% 14.5%      360 105 109 62 180 Mix A Lead Slag          ie 29%  2/3/89 5 80 kg High      200 54.8% 11.2% 34.0% 4.3% 15.3% 12.2% 14.0% 160 160 141 53 280 Mix A      Lead Slag          ie 100% 3/3/89 6 Continuing + 160 83.1% 16.9% -- --      20.3% 16.1% 14.2% 105 105 119 68 180 Mix B 30 kg High          ie 100%      3/3/89 Lead Slag 7 90 kg High 175 83.1% 16.9% -- -- 20.3% 16.1% 13.5%      115 100   91 55 140 Mix B Lead Slag          ie 87%  4/3/89 8 28 kg      Reverb 165 82.0% 18.0% -- -- -- 111.7%  22.0% 150 97  92 57 410 Mix B      Slag +           ie 65%  7/3/89 51 kg Fumes 9 Continuing 170 82.0% 15.0      -- -- -- 43.9% 19.6%  45 45  50 67 240 Mix B from Run 8          ie 100%      7/3/89 10 50 kg Slag 120 82.0% 13.0% -- -- -- 24.1% 16.5% 185 145  243      101  330 Mix B from Run 9 +          ie 78%  8/3/89 12 kg Fume 11 50 kg      Slag 110 54.6% 12.1% 33.3% 4.0% --  4.4% 13.5% 110 110  180 98 380 Mix A      from Run 9 +          ie 100% 10/3/89 12 kg Fume 12A Continuing 210      54.7% 11.8% 33.5% 4.1%  4.3%  3.4% 12.7%  50 50  79 95 300 Mix A from      Run 11          ie 100% 10/3/89 12B Continuing 300 54.8% 11.2% 34.0%      4.3% 15.3% 12.2% 12.0% 45 45  60 80 390 Mix A from Run 12A          ie      100% 10/3/89 13 35 kg Slag 120 82.0% 18.0% -- -- --  1.8% 13.0% 80 80      202 152  350 Mix B from Run 9 +          ie 100% 13/3/89 8 kg Fume 14      Continuing 100 82.4% 17.6% -- --  8.4%  6.7% 13.0% 45 32  72 135  185      Mix B from Run 13          ie 71%  13/3/89 15 40 kg Slag 130 54.6% 12.1%      33.3% 4.0% --  3.1% 13.0% 95 95 251 159  400 Mix A from Run 9 +      ie 100% 14/3/89 4 kg Fume 16A Continuing 130 54.7% 11.7% 33.6% 4.2% 6.6%       5.3% 13.0% 60 57 132 139  295 Mix A from Run 15 14/3/89 16B Continuing      295 54.8% 11.2% 34.0% 4.3% 15.3% 12.2% 13.0% 35 35  70 120 350 Mix A      from Run 16A          ie 100% 14/3/89 17 Continuing 185 82.4% 17.6% --      --  8.5%  6.8% 13.0% 120 90 204 136 495 Mix B from Run 16B          ie      75%        14/3/89      PRODUCTS STEADY STATE STEADY STATE  Bag LANCE AIR RATE OIL RATE  FUME      RATE  Tapped House Oxygen Rate Excess  g/kg New SMELTING Dry % of New      Material  RUN Slag Fume Content Nm.sup.3 Oxygen kg/hr Material TEMP  By      Mass  NUMBER (kg) (kg) (vol %) kg Feed) Content (Total) (See Note)      (°C.) Measured Balance COMMENTS        1 68 46 21 2.74 1.1%  9.3 39 1170 to 15% -- Steady operation. Mix A          1200 27/2/89 2 213 55 21 2.64 1.2%  7.2 16 1200 to 22% -- Steady      operation. Oil consumption Mix A        1210   dropping as furnace heats      up. 27/2/89 3 75 + 41 21 4.08 0.9% 14.4 115  1160 to 64% -- Irregular      operation as plant air low, Mix A 82 in       1180   =) Several stoppages       to heat bath. 1/3/89 offtake 4 46 + 80 21 4.31 1.7 12.0 103  1180 to      73% -- Irregular operation as plant air low, Mix A 54 in       1200   =)      Several stoppages to heat bath. 2/3/89 5 113 100  21 4.67 1.0% 12.3 127      1220 to 71% -- Reasonably steady operation. Mix A        1240   Rates      low as plant air still low. 3/3/89 6 60% + 62 21 4.67 2.2% 13.5 116      1170 to 52% -- Plant air low. Thick slag on furnace Mix B 100 mm      1250   bottom covering cermet. =) large 3/3/89 sludge          temperatur      e and oil fluctuations 7 52 + 48 21 5.53 2.1% 15.6 183  1150 to 53% --      Plant air low. Mix B wall       1180   Accretion buildup on walls.      4/3/89 accretion 8 110 53 21 4.47 1.5% 19.8 250  1160 to 58% -- Plant      air low, =) irregular operation Mix B        1190   to start but      improving towards end of 7/3/89           run 9 148 14 21 3.68 3.2% 16.5      164  1190 to 28% 44%   Steady operation. Mix B        1210 7/3/89 10 110      53 21 2.40 3.0% 15.9 102  1140 to 22% 23%   Lance change from 30 mm to      40 mm at Mix B        1180   start of run due to high lance back- 8/3/89                pressures. Steady operation after            first 40 kg feed.      11 74 54 21 2.52 3.0% 13.2 77 1170 to 30% 3.4% Steady operation. Mix A           1200   Good hygiene. 10/3/89 12A 145 kg  9 21 2.35 3.0%  8.7 33      1188 to 11% 2.8% Steady operation. Mix A total       1200   Good      hygiene. 10/3/89 for          Approximately 50 mm stable foam. 12B 12A &       7 21 3.96 3.0% 11.1 63 1185 to 12% 10.0%  Fairly steady operation.      Hygiene Mix A 12B       1200   deterioration at this higher coke rate.      10/3/89           Approximately 100 mm stable foam. 13 138  6 35 0.82      3.0% 13.2 63 1180 to  5% 8.3% Burner Out Overnight, Bags Oiled =) Mix B            1210   Re-Bagged Oxygen problems at start. 13/3/89           bath      held approx 1 hr.            Steady operations once started.      Good hygiene. 14 76  9 35 0.93 3.0%  6.0 16 1170 to 12% 7.2% Downtime to      repair oxygen line and Mix B        1220   heat bath. Lost oxygen      pressure at 13/3/89           end of run. Good hygiene. 15 147  5 35      0.98% 3.0% 12.0 52 1150 to  2% 3.2% Steady operation. Mix A        1180       Good hygiene. 14/3/89 16A 105 kg  3 35 1.08 3.0%  6.0 16 1150 to  2%      5.5% Steady operation apart from 3 min Mix A total       1180   downtime      to change oxygen bottles. 14/3/89 for          Good hygiene. 16B 16A &      1 35 1.33 3.0%  2.1 -14  1160 to  2% 12.6%  Steady operation. Mix A 16B           1180   Approximately 100 mm stable foam. 14/3/89 17 200  9 35 1.00      3.0%  7.2 25 1170 to  5% 8.4% Initial hygiene problems. Mix B      1210   25 min downtime for lance change - 14/3/89           burnt      *Recycle lead sinter plant dust.      **Lead containing dust from copper coverter electrostatic precipitator.      Note.      Specific Cil Rate, Corrected for Furnace Heat Losses.      (equivalent to 5.6 kg oil/hr for air)      (equivalent to 3.8 kg oil/hr at 35:02)

                                      TABLE 3     __________________________________________________________________________                  2     ASSAYS (wt %)     RUN     NUM-           Ag  Au     BER         Pb (ppm)                        (ppm)                            Zn Fe Cu S   SiO2                                            Al2O3                                                CaO                                                   MgO                                                      As Sb Cd  Bi     __________________________________________________________________________     1    Starting Bath                 42.1                     857                        --  8.2                               21.9                                  0.85                                     <0.01                                         9.2                                            2.1 1.3                                                   0.86                                                      0.04                                                         0.21                                                            0.004                                                                <0.001     Mix A          Final Slag                 8.6                    1700                        27.7                            7.5                               30.3                                  3.19                                     0.25                                         21.2                                            3.2 6.5                                                   1.16                                                      0.23                                                         0.94                                                            0.001                                                                0.003     27/2/89          Final Fume                 61.4                    2650                        1.5 7.70                               0.47                                  0.10                                     5.40                                         0.60                                            0.12                                                0.12                                                   0.12                                                      2.68                                                         0.52                                                            1.090                                                                0.510     2    Starting Bath                 8.6                    1700                        27.7                            7.5                               30.3                                  3.19                                     0.25                                         21.2                                            3.2 6.5                                                   1.16                                                      0.23                                                         0.94                                                            0.001                                                                0.003     Mix A          Final Slag                 8.1                    1620                        20.3                            6.8                               29.2                                  3.01                                     0.44                                         21.9                                            3.1 7.3                                                   0.95                                                      0.23                                                         1.02                                                            0.002                                                                0.002     27/2/89          Final Fume                 59.2                    2024                        2.3 6.60                               1.94                                  0.24                                     5.00                                         1.80                                            0.29                                                0.43                                                   0.27                                                      2.56                                                         0.40                                                            1.140                                                                0.470     3    Starting Bath                 13.0                    1060                        20.7                            6.9                               27.7                                  3.22                                     0.02                                         19.8                                            3.0 6.0                                                   1.59                                                      0.06                                                         0.85                                                            0.001                                                                0.002     Mix A          Final Slag                 16.6                    1140                        17.8                            7.4                               25.4                                  2.96                                     0.37                                         19.3                                            2.8 5.8                                                   1.36                                                      0.22                                                         0.87                                                            0.022                                                                0.037     1/3/89          Final Fume                 57.6                    6400                        17.7                            6.10                               1.29                                  0.27                                     5.40                                         1.90                                            0.22                                                0.28                                                   0.21                                                      3.69                                                         0.52                                                            1.460                                                                0.600     4    Starting Bath                 29.3                     640                        4.6 8.3                               25.9                                  1.66                                     <0.01                                         13.4                                            2.5 2.8                                                   1.26                                                      0.03                                                         0.47                                                            <0.001                                                                0.001     Mix A          Final Slag                 19.2                     660                        6.3 8.7                               25.7                                  2.39                                     <0.01                                         18.6                                            2.7 4.7                                                   1.34                                                      0.10                                                         0.77                                                            0.008                                                                0.023     2/3/89          Final Fume                 59.9                    4437                        26.5                            5.80                               0.76                                  0.19                                     5.80                                         0.10                                            0.13                                                0.12                                                   0.12                                                      3.12                                                         0.49                                                            1.340                                                                0.470     5    Starting Bath                 27.2                     535                        1.4 7.7                               25.1                                  1.75                                     <0.01                                         15.3                                            2.6 3.3                                                   1.72                                                      0.01                                                         0.49                                                            <0.001                                                                0.001     Mix A          Final Slag                 14.6                     445                        4.8 7.7                               26.3                                  2.92                                     <0.01                                         20.3                                            3.0 5.9                                                   1.77                                                      0.11                                                         0.92                                                            0.001                                                                0.001     3/3/89          Final Fume                 61.0                    4582                        20.9                            6.60                               0.30                                  0.19                                     5.30                                         -- 0.11                                                0.08                                                   0.07                                                      3.64                                                         0.39                                                            1.250                                                                0.520     6    Starting Bath                 36.1                     885                        2.0 8.1                               24.2                                  1.25                                     <0.01                                         10.9                                            2.3 2.1                                                   0.99                                                      0.06                                                         0.36                                                            0.004                                                                0.003     Mix B          Final Slag                 15.1                     220                        6.1 10.1                               24.8                                  2.59                                     0.02                                         18.1                                            3.4 7.7                                                   1.51                                                      0.21                                                         1.04                                                            0.005                                                                0.015     3/3/89          Final Fume                 62.4                    3227                        19.9                            3.60                               0.12                                  0.14                                     6.00                                         -- 0.10                                                0.04                                                   0.06                                                      3.81                                                         0.36                                                            1.380                                                                1.100     7    Starting Bath                 39.4                     770                        1.5 8.6                               23.4                                  1.10                                     0.04                                         9.3                                            2.2 1.6                                                   0.81                                                      0.06                                                         0.30                                                            <0.001                                                                0.002     Mix B          Final Slag                 28.4                     550                        5.7 9.8                               20.5                                  2.59                                     <0.01                                         14.0                                            2.7 5.3                                                   1.12                                                      0.28                                                         0.88                                                            0.010                                                                0.076     4/3/89          Final Fume                 66.1                    3226                        28.2                            1.87                               0.05                                  0.12                                     5.60                                         -- 0.07                                                0.03                                                   0.03                                                      3.82                                                         0.31                                                            1.310                                                                1.050     8    Starting Bath                 27.2                     700                        3.2 6.2                               23.1                                  0.79                                     <0.01                                         15.8                                            2.9 3.3                                                   1.46                                                      0.21                                                         0.36                                                            0.008                                                                0.014     Mix B          Final Slag                 39.2                    1575                        13.6                            8.2                               17.1                                  1.39                                     <0.01                                         11.0                                            1.8 3.7                                                   1.06                                                      0.91                                                         0.66                                                            0.152                                                                0.200     7/3/89          Final Fume                 60.9                    4880                        29.2                            1.34                               0.07                                  0.11                                     5.30                                         -- 0.31                                                0.04                                                   0.04                                                      2.78                                                         0.25                                                            3.600                                                                0.460     9    Starting Bath                 39.2                    1575                        13.6                            8.2                               17.1                                  1.39                                     <0.01                                         11.0                                            1.8 3.7                                                   1.06                                                      0.91                                                         0.66                                                            0.152                                                                0.200     Mix B          Final Slag                 39.8                    1435                        15.6                            8.2                               15.5                                  1.71                                     <0.01                                         10.7                                            1.8 4.4                                                   0.91                                                      1.04                                                         0.78                                                            0.098                                                                0.260     7/3/89          Final Fume                 61.4                    4262                        28.4                            1.23                               0.10                                  0.11                                     5.70                                         -- 0.11                                                0.04                                                   0.06                                                      3.63                                                         0.25                                                            3.510                                                                0.710     10   Starting Bath                 35.3                    1240                        9.7 8.4                               30.5                                  1.74                                     <0.01                                         11.4                                            2.1 4.1                                                   1.53                                                      0.42                                                         0.72                                                            0.015                                                                0.102     Mix B          Final Slag                 40.8                    1065                        12.8                            8.9                               12.5                                  2.84                                     <0.01                                         11.3                                            2.0 6.6                                                   0.82                                                      0.84                                                         1.14                                                            0.011                                                                0.076     8/3/89          Final Fume                 62.0                    3635                        23.5                            1.25                               0.02                                  0.08                                     5.80                                         -- 0.08                                                0.02                                                   0.08                                                      3.89                                                         0.21                                                            2.570                                                                1.710     11   Starting Bath                 42.2                    1100                        9.8 8.3                               17.0                                  1.41                                     <0.01                                         10.2                                            1.8 3.9                                                   0.92                                                      0.70                                                         0.65                                                            0.020                                                                0.102     Mix A          Final Slag                 41.4                    2016                        13.6                            8.2                               15.7                                  2.05                                     0.01                                         11.1                                            1.5 4.1                                                   0.56                                                      1.10                                                         0.86                                                            0.060                                                                0.126     10/3/89          Final Fume                 59.6                    7042                        12.1                            0.79                               0.03                                  0.09                                     4.60                                         -- 0.10                                                0.02                                                   0.02                                                      5.50                                                         0.31                                                            3.210                                                                0.880     12A  Starting Bath                 41.4                    2016                        13.6                            8.2                               15.7                                  2.05                                     0.01                                         11.1                                            1.5 4.1                                                   0.56                                                      1.10                                                         0.86                                                            0.060                                                                0.126     Mix A          Final Slag                 33.0                    2216                        17.3                            8.1                               16.1                                  2.23                                     0.03                                         12.2                                            1.6 4.6                                                   0.53                                                      1.20                                                         0.91                                                            0.127                                                                0.170     10/3/89          Final Fume                 60.6                    6269                        24.7                            1.33                               0.16                                  0.15                                     6.00                                         -- 0.12                                                0.05                                                   0.07                                                      3.42                                                         0.34                                                            2.940                                                                0.450     12B  Starting Bath                 38.0                    2216                        17.3                            8.1                               16.1                                  2.23                                     0.03                                         12.2                                            1.6 4.6                                                   0.53                                                      1.20                                                         0.91                                                            0.127                                                                0.170     Mix A          Final Slag                 37.6                    2192                        17.4                            8.1                               16.1                                  2.15                                     <0.01                                         11.8                                            1.6 4.5                                                   0.61                                                      1.20                                                         0.89                                                            0.160                                                                0.181     10/3/89          Final Fume                 58.2                    6687                        25.3                            0.85                               0.09                                  0.18                                     6.60                                         -- 0.11                                                0.04                                                   0.07                                                      3.23                                                         0.35                                                            2.960                                                                0.460     13   Starting Bath                 38.0                    1038                        10.4                            8.7                               19.5                                  1.36                                     <0.01                                         10.4                                            1.9 3.3                                                   0.86                                                      0.55                                                         0.62                                                            0.021                                                                0.070     Mix B          Final Slag                 39.7                    1457                        24.7                            8.1                               12.9                                  2.40                                     0.06                                         10.8                                            1.9 6.4                                                   0.60                                                      1.30                                                         1.11                                                            0.193                                                                0.221     13/3/89          Final Fume                 57.2                    4514                        25.8                            1.09                               0.07                                  0.14                                     5.30                                         -- 0.15                                                0.05                                                   0.04                                                      4.14                                                         0.30                                                            3.830                                                                1.220     14   Starting Bath                 39.7                    1457                        24.7                            8.1                               12.9                                  2.40                                     0.06                                         10.8                                            1.9 6.4                                                   0.60                                                      1.30                                                         1.11                                                            0.193                                                                0.221     Mix B          Final Slag                 34.9                    1405                        26.2                            8.9                               14.4                                  2.78                                     0.01                                         12.1                                            2.3 7.6                                                   0.71                                                      1.34                                                         1.26                                                            0.124                                                                0.231     13/3/89          Final Fume                 60.3                    3831                        36.0                            1.49                               0.17                                  0.13                                     5.50                                         -- 0.14                                                0.06                                                   0.05                                                      3.63                                                         0.29                                                            3.090                                                                0.850     15   Starting Bath                 26.8                     793                        11.2                            9.0                               22.5                                  2.26                                     <0.01                                         13.2                                            2.6 5.9                                                   1.28                                                      0.47                                                         0.93                                                            0.068                                                                0.109     Mix A          Final Slag                 30.7                    2016                        18.1                            8.2                               18.1                                  2.65                                     <0.01                                         14.1                                            1.9 5.5                                                   0.64                                                      0.98                                                         1.07                                                            0.101                                                                0.193     14/3/89          Final Fume                 57.7                    5499                        41.0                            2.18                               0.64                                  0.18                                     4.90                                         -- 0.09                                                0.05                                                   0.05                                                      5.44                                                         0.35                                                            2.640                                                                0.770     16A  Starting Bath                 30.9                    2016                        18.1                            8.2                               18.1                                  2.65                                     <0.01                                         14.1                                            1.9 5.5                                                   0.64                                                      0.98                                                         1.07                                                            0.101                                                                0.193     Mix A          Final Slag                 30.0                    2038                        15.3                            8.4                               18.9                                  2.80                                     <0.01                                         14.6                                            1.9 5.6                                                   0.60                                                      0.95                                                         1.08                                                            0.102                                                                0.199     14/3/89          Final Fume                 58.0                    5967                        41.2                            2.13                               0.87                                  0.25                                     4.80                                         -- 0.15                                                0.13                                                   0.09                                                      5.18                                                         0.41                                                            2.440                                                                0.730     16B  Starting Bath                 30.0                    2038                        15.3                            8.4                               18.9                                  2.80                                     <0.01                                         14.6                                            1.9 5.6                                                   0.60                                                      0.95                                                         1.08                                                            0.102                                                                0.199     Mix A          Final Slag                 30.2                    2030                        14.9                            8.2                               18.0                                  2.66                                     <0.01                                         15.5                                            1.9 5.5                                                   0.58                                                      0.89                                                         1.06                                                            0.131                                                                0.213     14/3/89          Final Fume                 57.6                    7330                        27.1                            1.28                               0.37                                  0.25                                     4.50                                         -- 0.09                                                0.07                                                   0.02                                                      6.34                                                         0.49                                                            2.810                                                                0.740     17   Starting Bath                 30.2                    2030                        14.9                            8.2                               18.0                                  2.66                                     <0.01                                         15.5                                            1.9 5.5                                                   0.58                                                      0.89                                                         1.06                                                            0.131                                                                0.213     Mix B          Final Slag                 32.7                    1289                        17.3                            9.0                               15.7                                  2.81                                     <0.01                                         13.0                                            2.3 7.2                                                   0.85                                                      0.90                                                         1.21                                                            0.105                                                                0.260     14/3/89          Final Fume                 58.0                    4486                        36.9                            1.77                               0.85                                  0.21                                     5.60                                         -- 0.08                                                0.06                                                   0,.08                                                      4.18                                                         0.35                                                            2.820                                                                0.720     Note. Average Steady State Slags     are for Mix A:                 31.1                    2843                        23.2                            7.6                               15.8                                  2.45                                     0.01                                         11.6                                            1.7 4.35                                                   0.48                                                      2.25                                                         0.93                                                            0.675                                                                0.329     Mix B:      40.4                    1603                        31.6                            7.1                               9.1                                  2.57                                     0.01                                         8.9                                            1.9 6.03                                                   0.57                                                      2.27                                                         1.02                                                            1.060                                                                0.393     __________________________________________________________________________

EXAMPLE 2

Consideration of the 250 kg scale testwork has resulted in the flowsheet for the production plant shown in FIG. 1.

This proposed ISASMELT plant will smelt either Mix A or B material, or any combination thereof, and two specific scale up options are considered in this example:

Option 1: Smelt 4 tph of by-product lead dusts together with 2 tph of low lead pyritic concentrate, giving a total treatment rate of 3600 tonnes per month at an availability of 80%.

Option 2: Smelt only lead dusts at a rate of 4 tph, giving a treatment rate of 2400 tonnes per month at an availability of 80%.

A budget estimate for an ISASMELT plant has been developed and is shown in Table 4 and the cost estimate in Tables 5 and 6.

                  TABLE 4     ______________________________________     PROCESS SUMMARY                Option 1    Option 2     ______________________________________     Material     MIX A         MIX B                  54.7% recycle 82% recycle                  lead dust     lead dust                  12.0% converter                                18% converter dust                  dust                  33.3% pyritic                  concentrate     Treatment rate, ie @                  6 tonnes/hr   4 tonnes/hr     80% availability                  3600 tonnes/month                                2400 tonnes/month     Lead slag product                  5.0 tonnes/hr 3.2 tonnes/hr                  (31% Pb,      (40% Pb,                  <0.05% S)     <0.05% S)     Operating    1170 to 1190° C.                                1170 to 1190° C.     temperature     Lance air (35% 0.sub.2)     Blower @ 150 kPa                  1.33 Nm.sup.3 /s                                0.75 Nm.sup.3 /s     Oxygen @ 150 kPa                  0.29 Nm.sup.3 /s                                0.16 Nm.sup.3 /s                  (36 tpd)      (20 tpd)     Coke breeze  650 kg/hr     510 kg/hr     Natural gas  125 Nm.sup.3 /hr                                125 Nm.sup.3 /hr     (standby burner)                  10% duty cycle                                10% duty cycle     Silica flux  240 kg/hr     --     Recycle fume 300 kg/hr     200 kg/hr     (5% of feed)     Spray cooling water                  1.5 liters/s  1.1 liters/s     Furnace offgas     Wet          4.9 Nm.sup.3 /s                                3.6 Nm.sup.3 /s     Dry          2.6 Nm.sup.3 /s @ 8.4%                                2.0 Nm.sup.3 /s @ 4.9%                  SO.sub.2      SO.sub.2     Dust loading 10 g/m.sup.3  9 g/m.sup.3     (wet gas @ 200° C.)     Offgas after 2.8 Nm.sup.3 /s @ 7.8%                                2.2 Nm.sup.3 /s @ 4.5%     process      SO.sub.2      SO.sub.2     gas baghouse (dry basis)   (dry basis)     ______________________________________

                  TABLE 5     ______________________________________     BUDGET CAPITAL ESTIMATE     FOR ISASMELT PLANT                          Australian     ______________________________________     Mixer                  $  100 000     Feed Conveyor          50 000     ISASMELT vessel (refractory lined,                            ↓↓↓ ↓↓↓     1.0⊥⊥ ID)     Lance handling         75 000     Plenum                 50 000     Control room           75 000     Process air blower (400 kW)                            200 000     Process air line       75 000     Process gas fluo (150M)                            75 000     Structural steel       350 000     Foundations            150 000     SUBTOTAL               $2 000 000     Process gas baghouse   200 000     Process gas fan        40 000     Chain conveyor         75 000     SUBTOTAL               $2 315 000     Instrumentation @ 10%  231 500     Electrics @ 10%        231 500     Engineering design @ 20%                            463 000     SUBTOTAL               $3 241 000     Contingency @ 30%      972 000     Total budget capital estimate                            AU$4 213 000     ______________________________________      Exclusions:      Feed bins and weighers      Hygiene ventilation requirements      Product handling, i.e., slag granulation or casting      Provision of services

                  TABLE 6     ______________________________________     PART 1     Operating Cost Estimate for Option 1     Smelting 6 tonnes/hr of Lead Dusts and Concentrate     at 80% availability     ______________________________________                                   ANNUAL                                   COST     ITEM      BASIS FOR ESTIMATE  AUD (1000)     ______________________________________     Labour    1/2 Metallurgist/Supervisor                                    40               12 Operators (ie 3 per shift)                                   568     Oxygen    0.29 Nm.sup.3 /s by 80%                                   978     Coke breeze               650 kg/hr by 80%    170     Flux      240 kg/hr by 80%     12     Natural gas               125 Nm.sup.3 /hr by 10%                                    19               (heating and holding)     Engineering               Labour: 1 Fitter     47     maintenance               Materials: Based on current                                   100               ISASMELT usage     Other stores               Based on current ISASMELT                                   150               usage     Refractories               50 tonnes Chrome-Magnesite                                   200               bricks (1 year life) plus               Launders, etc. (labour and               materials     Baghouse bags               630 m.sup.2 Ryton bags                                    43               (bag life of 1 year)     Compressed air               Based on current ISASMELT                                    50               usage               (baghouse and general)     Power     Total power usage 500 kW                                   178     Fresh Water               6 m.sup.3 /hr by 90%                                    5               (flue spray and general)     Transport Based on material moved                                    82     TOTAL COST                    2640     UNIT COST PER TONNE OF LEAD     33800 tonnes slag produced pa @ 31% Pb                               AUD252     ______________________________________     PART 2     Operating Cost Estimate for Option 2     Smelting 4 tonnes/hr of Lead Dusts at 80% availability     ______________________________________                                   ANNUAL                                   COST     ITEM      BASIS FOR ESTIMATE  AUD (1000)     ______________________________________     Labour    1/2 Metallurgist/Supervisor                                    40               12 Operators (ie 3 per shift)                                   568     Oxygen    0.16 Nm.sup.3 /s by 80%                                   540     Coke breeze               510 kg/hr by 80%    133     Flux      None required        0     Natural gas               125 Nm.sup.3 /hr by 10%                                    19               (heating and holding)     Engineering               Labour: 1 Fitter     47     maintenance               Materials: Based on current                                   100               ISASMELT usage     Other stores               Based on current ISASMELT                                   150               usage     Refractories               50 tonnes Chrome-Magnesite                                   200               bricks (1 year life) plus               Launders, etc. (labour and               materials)     Baghouse bags               630 m.sup.2 Ryton bags                                    32               (bag life of 1 year)     Compressed air               Based on current ISASMELT                                    50               usage               (baghouse and general)     Power     Total power usage 500 kW @                                   178               USDO.030/kWH     Fresh Water               5 m.sup.3 /hr by 90%                                    4               (flue spray and general)     Transport Based on material moved                                    54     TOTAL COST                    2115     UNIT COST PER TONNE OF LEAD     21700 tonnes slag produced pa @ 40% Pb                               AUD244     ______________________________________

The maximum smelting rates used during the 250 kg testwork were 150 to 160 kg/hr for Mix A and B materials, indicating that a furnace of 1.3 m internal diameter would smelt in excess of 6 tonnes/hr. For greater operating flexibility, however, a diameter of 1.8 m is preferred for both Options 1 and 2.

The lance air rates used during the 250 kg testwork are calculated to give a slight oxygen excess, possibly 3%, in the offgas in addition to fully oxidizing the feed and burning the coke and oil. An excess oxygen concentration has been shown from the Applicant's plant operating experience to provide optimum smelting conditions.

The smelting air rates enriched to 35% O₂ used during the trials were 0.421 Nm³ /kg of Mix A and 0.175 Nm³ /kg of Mix B, with a fuel combustion air rate of 5.09 Nm³ /kg of coke breeze. Scaling in proportion, the lance air requirement enriched to 35% O₂ on a production plant is 1.62 Nm³ /s for Option 1 and 0.91 Nm³ /s for Option 2.

The air component of these flowrates is 1.33 Nm³ /s for Option 1 and 0.75 Nm³ /s for Option 2. The air supply pressure is 150 kPa for low pressure lances, supplied from a single stage blower.

The oxygen consumptions to enrich the lance air to 35% O₂ are 36 and 20 tonnes per day for Options 1 and 2 respectively. Oxygen enrichment is used to increase the SO₂ concentration in the process offgas to aid SO₂ recovery in an acid plant and to reduce the offgas volumes.

During the 250 kg testwork, both coke breeze and oil were used together as fuels in varying proportions. In the final runs at 35% O₂, coke breeze additions up to 10 times the oil rate were used. Under these conditions, the furnace was over-coked and a stable foam approximately 100 mm thick formed on the bath surface. The foam did not cause problems and is not present when the furnace is correctly coked. The use of coke breeze as the sole fuel is preferred.

The fuel consumptions in coke equivalents during the trials at 35% O₂ were 90 kg coke breeze/dry tonne of Mix A and 100 kg coke breeze/dry tonne of Mix B, at 5% fume recycle and 15% moisture in the feed, after correcting for heat losses from the 250 kg rig. The furnace heat losses on the proposed plant are estimated to be a maximum of 0.5 MW which is equivalent to 110 kg of coke breeze per hour at 35% O₂.

The resultant fuel consumptions are 650 kg coke breeze/hr for Option 1 and 5l0 kg coke breeze/hr for Option 2.

The process gas at the top of the furnace, including 1 Nm³ /s ingress air, is shown in Table 7 below:

                  TABLE 7     ______________________________________              Option 1    Option 2              6 tonnes/hr Mix A                          4 tonnes/hr Mix B     ______________________________________     O.sub.2    9.4%          11.7%     N.sub.2    69.8%         70.3%     SO.sub.2   8.4%          4.9%     CO.sub.2   12.4%         13.1%     TOTAL      2.6 Nm.sup.3 /s                              2.0 Nm.sup.3 /s                (dry basis)   (dry basis)     ______________________________________

The spray water required to cool the gas to 200° C. is 1.5 liters/s for Option 1 and 1.1 liters/s for Option 2, giving total wet offgas volumes of 4.9 and 3.6 Nm³ /s respectively.

A dedicated process gas baghouse with integral fume return system enables the fume to be recycled directly to the ISASMELT furnace at the same rate as it is produced. This stabilises the process by automatically correcting any excursions and thus improves metallurgical control. This is especially preferred for the lower lead slags of Option 1.

Pulse air plus ingress for a reverse pulse baghouse is estimated to be 0.2 Nm³ /s for both Options 1 and 2. The gas volumes and SO₂ concentrations entering the acid plants will thus be 2.8 Nm³ /s at 7.8% SO₂ and 2.2 Nm³ /s at 4.5% SO₂ (both on a dry basis) for Options 1 and 2 respectively.

The fume rates during the 250 kg testwork were dependent upon the gas volume flushed through the bath. For lance air enriched to 35% O₂ the fume rate ranged from 5 to 10% by weight or the reed. Fuming tended to be slightly greater for Mix B material, in accordance with the higher PbO activity in the slag, but this difference was not significant at 35% O₂. On a full size plant the fume rate will be approximately 5% of the feed rate.

During the 250 kg testwork it was not possible to recycle the fume from the baghouse directly on the plant, instead the fume from previous runs was pelletized and added at the same rate as that measured at the baghouse.

As a result, the minor elements in the fume did not reach steady state except for antimony (Table 3). The steady state fume compositions are estimated as 0.4% Sb, 10% As, 1.5% Bi and 15% Cd for Mix A and 0.4% Sb, 10% As, 1.5% Bi and 20% Cd for Mix B.

Silver and gold in the fume did not reach steady state during the testwork. The steady state fume compositions are estimated as 8000 ppm Ag and 40 ppm Au for Mix A, and 5000 ppm Ag and 50 ppm Au for Mix B. Silver and gold were fully recovered once the fume was recycled.

The furnace may be either batch or continuously tapped, although due to the relatively low tonnages batch tapping is preferred to maintain reasonable slag flowrates and to minimize labour.

The holding capacity of a 1.8 m internal diameter furnace will allow 2.5 hours operation between taps for Option 1 and 4 hours for Option 2.

The slag may be either granulated or cast depending upon requirements.

Lance lives currently average 100 hour operating times.

The furnace is lined with chrome-magnesite wear bricks and based on current refractory lives a minimum period of one year is expected between rebricks.

Since the ISASMELT furnace is almost totally enclosed and a moist agglomerate feedstock is used good hygiene is likely to be achieved.

Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. 

We claim:
 1. A method for the treatment of a mixture of lead dusts and pyritic concentrates said method including the steps of:a) feeding a mixture of lead dust and pyritic concentrate to a furnace containing molten slag; b) smelting the mixture by injecting air or oxygen enriched air into the slag by means of a submerged lance to produce a lead bearing slag; c) recycling fume to the furnace; and d) periodically or continuously tapping the furnace to recover a lead containing slag.
 2. A method for the treatment of lead dusts, without pyritic concentrate, said method including the steps of:a) feeding lead dust to a furnace containing molten slag; b) smelting the dust by injecting air or oxygen enriched air into the slag by means of a submerged lance to produce a lead bearing slag; c) recycling fume to the furnace; and d) periodically or continuously tapping the furnance to recover a lead containing slag.
 3. A method in accordance with claim 1, wherein steps a), b) and c) are continuous.
 4. A method in accordance with claim 1 wherein coal, coke, oil or natural gas, or any combination thereof, may be used as a fuel.
 5. A method in accordance with claim 1 wherein flux is added to produce a fluid slag.
 6. A method in accordance with claim 1 wherein the fume is recycled to the furnace via a baghouse.
 7. A method in accordance with claim 6 wherein the baghouse is a dedicated unit with an integral fume return system.
 8. A method in accordance with claim 1 wherein the preferred operating temperatures range from 1150° to 1125° C.
 9. A method in accordance with claim 8 wherein the temperature is 1170° to 1190° C.
 10. A method in accordance with claim 1 wherein the slag contains less than 2% sulphur and the process offgas will have an SO₂ concentration of greater than 4% to aid SO₂ recovery at an acid plant.
 11. A method in accordance with claim 1 wherein the slag has a composition in the range 20 to 50% Pb, 10 to 20% Fe, 5 to 15% SiO₂ and 2 to 5% CaO, with typical residual impurities of up to 10% Zn, 3% Al₂ O₃ and 3% MgO.
 12. A method according to claim 2, wherein the slag has a composition in the range 20 to 50% Pb, 10 to 20% Fe, 5 to 15% SiO₂ and 2 to 5% CaO, with typical residual impurities of up to 10% Zn, 3% Al₂ O₃ and 3% MgO. 